1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling the same, and more particularly to an image forming apparatus that forms images on image support members and a method of controlling the same.
2. Description of the Related Art
Electrophotographic image forming apparatuses are equipped with four image forming stations that carry out electrophotographic processes to form color images, such as black, cyan, magenta, and yellow images, respectively. Each of the image forming stations forms an electrostatic latent image on a photosensitive drum as an image support member by radiating a laser beam from a laser or light from a light-emitting device such as an LED onto the photosensitive drum. The electrostatic latent image formed on the photosensitive drum is then developed by toner supplied from a developing device, so that a visible image (toner image) is formed on the photosensitive drum. The toner images of the respective colors are transferred in a superimposed state from the photosensitive drum onto a belt-like intermediate transfer member (intermediate transfer belt). The toner images thus transferred in the superimposed state are collectively transferred from the intermediate transfer member onto a recording material to form a color image thereon. It should be noted that the toner images of the respective colors may be directly transferred in the superimposed state onto the recording material without being transferred onto the intermediate transfer member. In such a case, the recording material is conveyed by a belt-like recording material conveying member (conveying belt).
The image forming apparatuses described above form a group of toner images (hereinafter referred to as “patch image”) having different toner densities on the intermediate transfer belt or the conveying belt with respect to each color so as to carry out image adjustment. The toner densities of such patch images (hereinafter referred to as “patch densities”) are detected by a density sensor. The detected patch densities are compared with the target density, and the comparison results are used as feedback on image forming conditions such as the amount of light exposure, developing bias, tone correction curve, and density correction table. Thus, the toner densities of color images which are formed on the recording material can be suitably controlled, so that color images with stable color tones can be obtained. Specifically, it is possible to prevent the densities of supplied toners from varying with changes in usage environment and usage conditions such as an increase in the number of prints and therefore prevent situations where color images cannot exhibit their original tone colors.
FIG. 12 is a view useful in explaining a patch image which is formed by the conventional image forming apparatuses.
As shown in FIG. 12, a patch image comprised of a plurality of toner images having different toner densities is formed with respect to each color. Each toner image (hereinafter referred to as “patch”) is a rectangle of 20 mm×20 mm and contiguous to other patches. The patch image of each color is comprised of, for example, eight patches required to control the tone correction curve for each color to the target density.
The density sensor detects the patch density at a plurality of points such as ten points on each patch so as to reduce the effects of unevenness in the accuracy of reading patches caused by the intermediate transfer belt or the conveying belt. For this reason, the average value of patch densities obtained by reading (hereinafter referred to as “sampling” also) at ten points is typically used as the reading result.
On the other hand, in controlling the toner density as described above, toners have to be consumed so as to form patch images and/or clean formed patches. This causes an increase in the running cost of the image forming apparatuses. To address this problem, four techniques as described below have been proposed.
The first technique compares a detected patch density with the target density and change the formation cycle of patch images which are subsequently formed (see Japanese Laid-Open Patent Publication (Kokai) No. H03-251878, for example). With this technique, by decreasing patch image formation cycles, toner consumption is reduced to curb the rise in running cost.
The second technique changes the lengths of patches (forming intervals) in the direction in which the patches are read using characteristics of tone correction curves. Specifically, patch forming intervals for high-density areas and low-density areas are set to be short (see Japanese Laid-Open Patent Publication (Kokai) No. H08-076527, for example). With this technique, toner consumption required to control toner density can be minimized to curb the rise in running cost.
The third technique changes the image densities of patches and the number of patches according to the circumstances surrounding an image forming apparatus, i.e. usage environment (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-198805, for example). With this technique, a desired tone can be maintained in a stable manner over a wide tone range, and the image densities of patches and the number of patches can be minimized to curb the rise in running cost.
The fourth technique sets the number of times a shadow area where the sensor's detection range is narrow to a large value relative to the range of densities to be detected (see Japanese Laid-Open Patent Publication (Kokai) No. H10-142863, for example). With this technique, the accuracy of correcting processing in a shadow area can be improved.
With the first and second techniques, however, even if the patch image formation cycles or the patch formation intervals are changed, the period of time required to control the toner density remains unchanged in one formation cycle. It is therefore impossible to prevent degradation in the image formation capability of an image forming apparatus.
Also, with the third technique, even if the image densities of patches and the number of patches are changed according to the usage environment of an image forming apparatus, the correction time required to control the toner density for each patch remains unchanged. It is therefore impossible to satisfactorily improve the image formation capability of an image forming apparatus.
Also, with the fourth technique, the number of times a shadow area is read is increased relative to the total number of times all the areas are read so as to improve the accuracy of correcting processing. Since the total number of times all the areas are read remains unchanged, it is impossible to reduce processing time and downtime. Also, to increase the number of times a shadow area is read, the shadow area has to be long, resulting in increased toner consumption.